1. Field of the Invention
The present invention relates to a construction of a dynamic electric machine such as a totally-enclosed fan-cooled machine.
2. Description of the Related Art
Referring to FIG. 1, a totally-enclosed fan-cooled dynamic electric machine has a stator which is composed of a substantially cylindrical stator frame 1, stator cores 2 provided on the inner peripheral surface of the stator frame 1 and stator coils 3 wound on these stator cores 2. The machine also has a rotor 5 carried by a rotor shaft 6 and disposed inside the stator. The rotor shaft 6 is rotatably supported by bearings 7 which in turn are supported by end brackets 4 fixed to both ends of the stator frame 1. A cooling outer fan 8 is fixed to an end of the rotor shaft 6 extended to the outer side of the stator frame 1. An outer fan cover 9 is fixed by means of bolts 11 to seats 10 which are provided on the end bracket 4.
This type of dynamic electric machines have suffered from problem in that unpleasant noise is generated during operation due to resonance of the outer fan cover 9 caused by various factors such as mass unbalance of the rotor 5, electromagnetic vibration of the motor including high-frequency component generated during driving of the machine through an inverter, and, when the machine is a geared motor, mechanical vibration caused by meshing gears.
The following methods have been proposed in order to obviate the above-described problem.